1. Field of the Invention
The present invention relates generally to enhancing the photosensitivity of optical waveguides, and more particularly to a method for increasing optical waveguide photosensitivity to radiation, such as, ultra-violet radiation.
While the present invention is subject to a wide range of applications, it is particularly well suited for providing persistent ultra-violet photosensitivity to optical waveguides and tailoring the refractive index profiles of such waveguides.
2. Technical Background
Gratings produced by ultra-violet (UV) induced refractive index changes in optical waveguides are well known in the art. Broadly speaking, such gratings generally fall into one of two categories. The first category of gratings known as a long period grating (LPG) is commonly formed by UV exposure through an amplitude mask. The second category known as a short period grating or fiber Bragg grating (FBG) is typically formed using two--beam interference. Generally speaking, the use of a phase mask to create the two--beam interference is currently recognized as a preferred method of "writing" an FBG into an optical waveguide. With respect to both of these categories of gratings, it has been found that the strength of the grating (i.e., the change in refractive index resulting from UV exposure relative to the optical waveguide refractive index prior to UV exposure) is dependent upon, inter alia, the photosensitivity of the optical waveguide.
Gratings are typically formed in doped silica glasses commonly used in both planar waveguides and optical waveguide fibers which in turn are commonly used in opto-electronic components and other devices of telecommunication systems. It has been found that the sensitivity of optical waveguides containing these doped silica glasses to UV radiation can be enhanced by a hydrogen treatment. Early proposed H.sub.2 sensitization treatments involved exposing a germania doped silica glass to H.sub.2 at a relatively high temperature, typically at least 400.degree. C. Such high temperatures proved detrimental to germania doped optical waveguide fibers, as well as to other optical waveguide fibers containing other dopants. In view of the fact that optical waveguide fibers are typically coated with a polymeric coating material as part of the draw process, such extreme temperatures were found to destroy or at least severely damage the fiber coatings. Moreover, such high temperature sensitization treatments typically increased the attenuation associated with the fiber and/or weakened the fiber itself. In view of these shortcomings, improved H.sub.2 loading techniques have been developed. As a result, the photosensitive response of standard optical waveguide materials has been dramatically enhanced.
In one such technique, an SiO.sub.2 --based optical waveguide is exposed to an H.sub.2 atmosphere (partial pressure greater than one atmosphere) at low temperature (at most 250.degree. C.) for a period of days or weeks, such that the H.sub.2 diffuses into the optical waveguide. Such H.sub.2 loaded optical waveguides are then exposed to UV radiation to increase the refractive index of the irradiated portion of the optical waveguide, thus forming a grating. Normalized index changes on the magnitude of 5.times.10.sup.-5 or 10.sup.-4 have been achieved for practical devices using this technique while index changes as high as 10.sup.-2 have been reported for certain specialty fibers having less practical application. It was found that the index changes could persist substantially indefinitely provided the waveguide was not heated, and that at least a significant fraction of the change could survive moderate heating (e.g., less than or equal to 400.degree. C.).
In a more recent development, it has been found that the index of refraction of a germania doped optical waveguide fiber can be increased by treating the fiber with hydrogen and applying heat. The glass is exposed to hydrogen at a pressure in the range 14-11,000 p.s.i. and a temperature in the range 21.degree. to 150.degree. C. until sufficient hydrogen is diffused into the fiber. The fiber is then subjected to heat in excess of about 500.degree. C. using a flame or infrared radiation for a period of a second or less. This technique results in a substantial and long-lived increase in the normalized refractive index. Flame heating of a single mode germania doped optical waveguide fiber has produced normalized index changes (.DELTA.n/n) of 4.times.10.sup.-3.
Both of the above-described techniques rely on hydrogen loading to provide the required optical waveguide photosensitivity. Once loaded, the waveguide is then exposed to either UV radiation or heat to change the refractive index of the waveguide. With the exception of annealing to stabilize the index change, no further processing steps are contemplated by the above-described techniques. Accordingly, devices produced by these techniques have refractive index characteristics which can vary substantially due to, among other things, variations in fiber geometry, and/or the intrinsic photosensitivity of the fiber. Thus, although the above-described techniques can produce significant refractive index changes, there is little control over the accuracy of those changes. As a result, only a small percentage of the devices produced by the above-described techniques are capable of adequately meeting the needs and requirements of the systems in which they will be used absent some additional processing.
This is especially true of grating devices. With respect to FBG devices, for instance, the above-described H.sub.2 loading techniques lack sufficient accuracy with reasonable yield to selectively remove only the desired wavelength or wavelengths. For LPG devices, variation in center wavelength results in a low yield of acceptable devices.
In view of the foregoing, there is a need for a method of enhancing the UV photosensitivity of an optical waveguide, which simplifies the grating manufacture process while increasing the yield of acceptable gratings. In addition, there is a need for an optical waveguide which retains significant photosensitivity following out diffusion of the loading gas such that significant changes in refractive index can be achieved by exposing the waveguide to additional radiation treatments. Moreover, there is a need for a method of controlling the photosensitivity of an optical waveguide on a spatial scale on the order of microns.